The type of material used to build the rack will have minimal bearing on how well it holds cages on top. That is simply an issue of getting the weight distributed to the sides rather than the top shelf.
I can't really give you any advice on this not knowing the exact footprint of the Boaphile, or if you're building your rack slightly oversize for thermostats, etc. You'll have to fabricate some simple spacer and/or cleat system to distribute the weight.
Regarding lightweight material to build the rack from, that's a tough one. I've built a handful or racks from plywood and they have worked fairly well. Not my favorite material for racks by any means, though.
I'll attach a picture of a very simple rack I threw together one day to quarantine a group of snakes. The boxes are fairly tall but do not have the same area as your Iris boxes.
The boxes in this picture are ~32" x 16" x 13". The rack is over 6' tall.
I cut the shelves from a piece of birch-laminated plywood and then cut the excess into the strips you see making up the vertical supports of the rack. I think I had to buy one 1x3 for a backstop as I did not have enough leftover for 4 verticals and a back.
But with the Iris boxes you'll need a lot more material and should have plenty of excess material for the supports.
This was supposed to be a temporary rack so that's why I limited myself to two vertical per side. Ideally this rack would have 3 verticals per side.
Also, my drill had died so when I built this I only used a couple of screws on the whole cage. The rest was assembled with wood glue and 2" nails. My plan was to go back and add screws later.
Alas, that never happened. Even with this less than ideal design, and being held together with little more than glue/nails, this rack survived several months of use, 800 miles in the back of a moving truck, and was still in one piece after spending a half a day submerged under water during a flood. The shelves swelled from the flood water to the point where the boxes would not slide, of course, but the rack was still standing.
Using plywood and the open sides and back saves a lot of weight. It does take more skill to cut and assemble, however. I highly recommend cutting dados/rabbets into the sides and back with this design. I did not but it would have made the whole project stronger and so much easier to assemble.
There are ways to build a much lighter rack than this. It involves building frames for shelves and using 1/8" material or screen over the frames. This is a lot of work and takes some skill and equipment. I do not recommend it unless one is a skilled woodworker and has access to a decent shop.
With a design like this one has to fill the gaps with styrene or stryrofoam unless to room is heated.
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