How involved you have to get for the spar is largely dependent on the wingspan and type of flying (how many G's you want it to survive
). I have 3DLabPrint's P-38 with 55" wing span and they don't really even have a spar and it is designed for plain PLA. The P-38 is an early design of theirs that uses these rather curvy vertical lines that go through the full wing thickness.
Their latest designs are more typical aircraft design where they have a fairly thick spar web with cap "sockets" so that you can go back after assembly and put in some other material for the spar caps... They suggest 1.5 mm diameter carbon fiber rods for the extremist flyers.
Some time ago I was working on a design that left a box section at the spar location. My plans were to make separate box beams with webs printed on the printer bed with +/- 45 degree layers, assemble them into the beam and then run carbon fiber tow for spar caps. This design was for a 10' foot span flying wing.
Here's a half-size portion of the wing I printed for proof of concept of the software.