One of the most important things to understand working with epoxy is that it really doesn't have very much structural strength in itself, unless it's combined with a good design and materials that introduce the needed structural properties (like fiberglass, carbon or kevlar).
Based on my experience, I would recommend trying to keep the tolerances between the cavity and the insert component relatively tight. However, to ensure a super robustness, I would recommend an assembly of hard foam and the insert, and than installing the completed assembly into the board. Also, in going this way, you increase the size of the integrated bearing surface, both internally in the foam and across the area of the lamination. If you have a Dremel tool, you can easily custom a cavity to suit. A router is truly unnecessary, as the cavity area will be filled and buried under the lamination structure, so any minor tolerance variations become a moot point visually.
As far as cure rates, the best choice lies in the way that you go. If a larger volume of epoxy is used, particularly if casting a larger area with just epoxy, go with the slowest cure. If things are done within specification (the mixture and temperature), you will achieve the desired result over time. Catalytic materials are very predictable, keeping in mind the contraints in the use process.
I didn't mention it earlier, but it important not to work the material beyond the recommended working time. This is quite important, as things can quickly go off the map on your project if you exceed the viable timeframe recommended. Only do as much work as reasonable in a single session. A cavalier approach on things can be a bummer, so using the 20 minute stuff and trying to proper time things can be quite risky.
However, based on my recommendations above (going with tight interference tolerances) you can use a quicker, or more regular, hardener that might also be used for laminations too. Really, if things are reasonably tight, you can lose the chopped up fiberglass and easily use microballons instead, or even use only epoxy if your fit is reasonable good. A significant volume of material creates heat, so plan accordingly.
Again, if you find quicker/hotter cure rates a problem, just go with the slower cure rate. Why have to rush things? Time can be on your side.
Two to three layers of fiberglass is sufficient. To gain a bit more dimension of the overall process, I would recommend checking out the Board Lady's website. Eva is a consummate expert, and fortunately, she has offered an incredible amount of sound information on her website. Really, a "must" review for the correct map to the job.
Last edited by steveC; 18th November 2007 at 08:49 AM.