Nice setup!
I have something pretty similar for my HF mini-mill. Used an 18"x25" (plated steel) oil drip pan under the mill, no risers, with the mill's Y axis parallel to the 18" dimension. The Y handwheel is set just
outside the lip of the pan.
I spread silicone adhesive on the bottom of the mill base before doing the final bolt-down, so that if I ever want to use major amounts of coolant, there won't be any leakage down the bolt holes.
This setup has served me very well for something like 5 or 6 years. The large pan is great for catching most or all of the swarf, no matter what I'm machining. As an added bonus, I've found that the extra area of the drip pan to the sides of the mill are
great places to use a Sharpie to note down X, Y and Z position information for repeated cuts, etc. Once the part is done, a quick wipe with alcohol gives me a "clean slate."
PS - one thing that kind of concerns me about your riser system is the relatively small diameter of the riser tubes you used. I'd be worried that, if you tightened the bolts much beyond snug, you could deform the MDF(?) underneath your pan. True, the washers will help spread the load a bit, but they can deform, too. Guess I'm anal in the same way you are