As the others have mentioned, CAD software is a powerful tool. It may seem daunting at first, but you get the hang of it over time.
It’s also generally a good idea to base your design around standardized parts, e.g. C-channels with holes in multiples of 5 and/or to compromise for prototypes (eg making an oversized robot).
If you REALLY, REALLY wanted, and have access to to a drill press, you can buy aluminum C-channel stock for about 15$ per 8ft from McMaster (shipping costs are going to be a nightmare though) and you’ll probablly still end up buying VEX C-channels because competition-legal.
You’re correct in that it’s a bad idea, but for the wrong reason. I worked out a 2x35 hole C-channel to cost about $0.53 in PLA and $0.44 in ABS.
(Volume of a 2x35 hole c-channel is 21.22cm³, density of PLA is ~1.25g/cm³, density of ABS is ~1.04g/cm³, and 1kg of filament is about $20)
The real reason it’s a bad idea is due to the resulting part having layers perpendicular to the face of the C-channel, which can easily shear apart. I printed some as a joke and they broke withing a week of being thrown around. I suppose you could reinforce the model with ribs, but it’d still be susceptible to shearing. And there’s also the need for a 3D printer to 3D print stuff.