If you're 3D printing, take advantage of the situation of rounding off corners a bit. This will also slightly reduce bowing when starting with thick wires. Well done.
I made a pair of OPTs using microwave oven laminates as well. I used paper masking tape to separate each layer, with an additional layer of paper grocery bag between the primary to secondary interleavings. I used two layers of the paper grocery bag for the E-I gap, and held the assembled trafo together using JB Weld epoxy.
The laminates for the two OPTs weren't identical, but they had the tongue width of an EI-125. I found some metric dimensioned bobbins on ebay with a square cross-section, and peeled off laminates until the tongues fit.
I was targeting 64 ohm primary : 8 ohm secondary. 264 turns primary, 94 turns secondary, 2.8:1 turns ratio. I did half the primary wound up (20ga), then all the secondary wound down (16ga), then the other half of the primary wound up, so that's 3 layers @ 44 turns/layer primary with masking tape on each layer, then paper and tape, then 4 layers at 24 turns/layer (minus 2 turns) secondary, then paper and tape, then another 3 layers @ 44 turns/layer, with some plastic tape to make it pretty.
First one: DCR primary: 2.3 ohms. Primary inductance: 133mH. Leakage inductance: 0.54mH
The second one had the same 2.8:1 turns ratio but was 250 : 89 turns due to me running out of secondary wire. Also the winding window was a bit tighter on these laminates.
Second one: DCR primary: 2.1 ohms. Primary inductance: 105mH. Leakage inductance: 0.43mH
Providing all this in case it helps inform your next designs.
I built them into Arch Nemeses and I think they sound quite good.
