How about laying down a strip of copper or aluminum foil on an oversized pad of (non-conductive?) varnish, letting that dry and then using the conductive pen (or solder a piece of jumper wire you've already got to it for your connection
) and then varnishing it again for protection?Digital Decoupage!
I'd use lemon gold leaf or variegated gold leaf, but I'm a crazy sign painter who apprenticed BG (before Gerber)/BC (before computers) who minored in Fine Art, about 1/2 of it screen printing (in printmaking classes 1-3) & classic training in most other disciplines. So you never know what kind of $*#% I'll come up with.
I like the notion of using a small strip of PCB laid down across something like your bus section on the debugged rev of your 3.5" form factor design. It'd be a salable, limited edition run of collectible 3.5" ProtoBoards.
To maximize initial prepaid orders, add in the (estimated/amortized) cost of a free 2.5" final production board
and maybe a 10% discount on any future 2.5" production board orders by your "investors." Then you'll have the $ to defray your development costs to date and the development capital for the 3.5" limited edition up front.
You'll have another "buy in point" for the collectible limited edition
version of the 2.5" board for those comrades who'd want to wait for your proof of concept success before sending you $.
Give your initial "investors" a big discount (and another free 2.5" final production board
based again on the the (estimated/amortized) cost of their orders for the 2.5" collectible limited edition
For development boards & limited edition revisions you'd:
____ add the daughtercard section(s) to your main design along the most cost effective (short or long) edge.
____ divide it from the main card by a pair of properly spaced trace strips placed as guidelines for scoring/snapping _________ or spaced at the kerf width of a bandsaw or coping saw blade for cutting a more complex shape.
_________ I've got the bandsaw to do the cuts and can advise you as to the proper radii for complex shape turns.
By doing this for both sides of the bus section of your 2.5" initial prototype would give you:
___ 4 or 6 layers of PCB for the bus section (or the entire board) on your 2.5" design
________ at a minimal 2 sided PCB price increase.
___ 2 layers of solder mask dividing the ground plane from the main board signals for insulation.
___ avoid using headers as board interconnects
___ use bits of wire as your physical and electrical interconnect for signal traces AND ground!
___ another spot for placing ICs for the final 4 layer PCB production boards & simplify trace routing
. . . whatever, it might work out,
my brain just shut down and it's time to get ready for work.