There's a lot that could be learned from experiments by those of us on the forum. We don't do enough controlled testing of variables. I plead guilty of that myself, when I was breeding: it's easy to decide to stick with what's already working well.
In this instance, anyone with cages large enough to permit two boxes could include two, one with each of the two types of materials. You'd have to make sure the boxes are identical and in some way assure that the boxes' distances from the heated end was not a factor.
Some readers here breed enough pairs that their results would have some statistical significance and not be merely anecdotal. For others, having someone on the forum collect the results from everyone running the same test would result in a large sample that would be meaningful.
Others might instead test two differently shaped lay boxes, or sizes of lay boxes, or boxes made of different materials, placement at hot or cool ends, or even the orientation of the box's opening to light in the housing. (to increase the sophistication of these tests, everyone testing two different materials, for example, should control so that the sphagnum has about the same amount of water absorbed in it – this could be measured by the change in weight from the spagnum dry to once it's been wetted; similarly the peat and sand mix should contain those two elements in the same percentages. The sizes of boxes will of course vary according to species, but a quick conversation on this forum could probably result in some agreement for a range of box size-to-snake species.)
The key of course is that in a given year each breeder test only a single variable. This shouldn't be disruptive to most because the test can always include one box the way the breeder's been doing things and then a second one with only one of the variables changed. The biggest problem might be that it requires animals be kept in housing large enough to accommodate two lay boxes.
The nice thing is that even if a few of your animals have already layed this year, they can simply be excluded from the data collection and you can begin your experiment now.