When printing parts for the excellent honeycomb wall storage system, I like the fit to be fairly snug - sometimes too snug for a simple press-fit to get them into the grid. When that happens, I reach for this simple tapper/set tool that I can use to hold the insert pieces in place and tap them firmly with a hammer to seat them.

I printed mine in PLA, with 5 layers at 0.3mm resolution and 10% rectilinear infill, but if you tend to be a brute with the hammer, you may want to beef that up a bit.