That's been a tricky print.
I'm new to the MK3 so maybe there's lots of room for improvement but the slopes with supports did come up pretty awful.
I did print the regular one first and the "flat bottom" one afterwards.
Sliced with Slic3r with the default supports provided and the MK3 defaults for PLA (that's 210º fan at 100%, maybe printing a bit cooler would help with the bridging).
What seems strange to me is that the support material that came off of the second plane is super smooth so the layers should have plenty of support to bridge and I did print a benchy just before that ( https://www.thingiverse.com/make:459062 ) that bridged all the gaps without shagging
My kids are delighted with the two planes.
Thanks a lot for the design!