Time for an update. I'm now completely done with all the fiberglass work. It was a pain, but likely only because of my inexperience.I did end up going with lycra. And I was disappointed at first because the first coating of resin only made the fabric hard in spots where it was contacting something solid. All the areas that were just backed by air took 2 heavy coats of resin to harden up sufficiently. I also experienced some sag, despite stretching it rather tightly. I had to cut out some areas that were wrinkled and/or sagging, and then patch those areas with fiberglass. I ended up putting 3-4 layers of fiberglass on the top half of the box, and up to 5 layers on the flatter areas. These boxes are VERY solid, I suspect the internal bracing helped in that regard. Even though I was just using the dowel rods to support the rings, the bracing support from them being 1/2" diameter and hot glued and then Liquid Nailed, it certainly couldn't have hurt.Another tip that I'll throw out there is the can of resin recommended approximately 20 drops of hardener per 1 oz. of resin. I tried that on a fairly hot day and got maybe 3 minutes working time. I then tried 10 drops of hardener per oz. of resin and still only got 5-8 minutes working time. I found that using 3 drops (yes, only 3) worked quite well. It gave me a good 15-20 minutes of working time before it gelled up, and it was solid but tacky in a few hours, and fully cured in around 12-18 hours, which worked perfectly for what I needed. It also helped because I could add a layer half a day later and not have to sand/scuff the fiberglass first.I found cloth to be much better than mat, as far as workability. I did not have one of those aluminum bubble buster rollers to roll out air pockets, but I wish I had gotten one. I had to patch a few air pockets here and there. So if you're planning a project out, include one of these in your tool kit if you can.After finishing the fiberglass, I took an angle grinder with 2" Medium ScotchBrite aluminum oxide discs and got the fiberglass smooth, basically just getting rid of rough spots that would require buildup of bondo to cover. One drawback of using cloth instead of mat is that if the edges of the fabric are missing strands in one direction, the strands in the opposite direction will stick out a little and may be rough once glassed in. The frayed ends don't mesh and lay as well, so use sharp scissors and cut slightly back away from edges that are frayed.Fast forward to today... I taped the opening of the ports and placed them inside the boxes. I then took two 70 lb bags of sand and scooped sand into the boxes, one US Cup at a time, and tallied how many cups it took to fill each box. One box was 135 cups, or 1.12793 cu. ft., the other was 81 cups, or .676758 cu. ft. Wow, I really wasn't expecting that big of a difference! So I modeled both of these volumes in Bassbox, and found the difference would be noticable. I also found that the .676 cu. ft. box really modeled very nicely. So now I need to make up .451 cu. ft. of difference in volume by filling the larger box with something.My tentative plan is to use expanding foam insulation, but I need to read around and make sure that will work, or if I need some special closed cell foam. I'd love to hear if any of you have any recommendations on this. So it looks like I'm going to spray some foam into a "brick" or two, measure to match the needed displacement volume, and then secure them in the box so that they don't move or rattle. I could also estimate needed foam, spray in that much plus a little more, fill with sand, determine how much foam needs cut away, cut away excess foam, remeasure, etc. But I'll likely just make bricks.Here's a screenshot from BassBox Pro showing the subs, enclosure (assuming both boxes will be the same .676 cu. ft.) and model showing how well one sub in one box (not both) will graph.Will post more updates as soon as possible. Once I get the volumes the same, I'll be installing them temporarily, just to sound test them for a few days. Then if I'm satisfied, I'll finish them up with body filler, primer, and paint. If you have any questions, or want pics of something, feel free to ask.