It is honourable to build something from scratch for academic purposes, this is indeed how I learned about SMPS's, only moving on to 'specific' devices later.
That said, given the slower switching speed you're using for your MOSFET's, I assume you're also significantly lowering the power of your prototype. Lower efficiency, means more energy dissipated as heat. At 70% efficiency (possibly lower without tweaks), 30% of your input will go out as heat, mostly in the switches. For 100W, that's 30W to kick out as heat!
With low current driving for the mosfets, they spend more time 'half on', acting as resistors, and getting very hot.
Of course, you can just draw small power and ramping it up until your heatsinks get too hot.
Another great reference for academic purposes is DIY inverters. These convert 12/24v to mains at roughly 50/60Hz. You can of course increase this frequency and rectify the output(s) and many use nothing but basic logic oscillators, albeit at lmited efficiency (and therefore, power).