You can get buck converters from ebay for cheap... for example,
https://www.ebay.ca/itm/200W-15A-DC-...0AAOSwr~lYrBBB for $10. There are plenty to choose from, and these ones can adjust the output voltage. You just need to wait for it to arrive from wherever it's coming from. Might be similar to your 24v one.
I doubt you need a diode, but it wont hurt to put one in. It will drop your output voltage a bit, and you may need to compensate.
Oh, and if all you are doing is EOC...unless you're coasting for 15 minutes at a time, I wouldn't worry about your existing starting battery to be able to keep up...unless you've gone and put a tiny lithium pack in or something. You should have plenty of reserve time in it to coast for a minute or two without a bother.