Keep it simple.
It took me way more then 8 dives to teach myself Sidemount. I started by trying to figure it out with a plate as well. I get not wanting to invest until your sure. You will want to loosen your shoulder straps and tighten your crotch strap to lower the waist belt. Also for Sidemount you want it all to fit more snug. Your going to need something to put on top of the wing to keep it under control. I made a flat plate out of acrylic that mounted with the STA.
Now. I quickly realized this sucked, but it was enough to hook me on Sidemount. My first Sidemount rig I built from DECO Sidemount parts.
This is all you need for rigging. Use loop bungees.
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1+ on the above.
I can't fault the OP for their approach - it pretty much follows my own path. HOWEVER, in retrospect, my advice is to START with sidemounting.com and then go from there. It is a miniscule money investment that will save lots of money and frustration later. Disclaimer: I have NO association with sidemounting.com except as a happy student.
Finally, to the OP, since you are a DIYer. Doing sidemount right does not have to be expensive. The harness is easy to DIY, the tank rigging the same. Go buy yourself the wing - or if you are really into DIY then look up my thread on how to make your own. Mine is over 5 years old and going strong.