Any suggestions how to model a custom chopper frame in Catia?

Its look like you had already done with most of design. ( I mean preparing splines ) You just need rib comment in Part design module to make them pipe frame. Because of some personal reasons i always preffer to use "sweep" command in GSD module rather than "Rib" in PD. Sweep comman providing many more options to handle with complex profiles. Then you can make them solid by "close surface" or " thicken" command. Iam adding my edition for your file. You can check my progress in "ONR_PARTS" body. I just tried to show its logic so steps are simple. You can have much better results with improving it. I wish that helps.

EDIT: Iam not sure whats problem about my data. I think reason is versiron differences. Iam using V5 R21 version of CATIA. Past versions like R20,R18,R17 couldnt open my file. So ive decided to provide more shots to describe my steps for data.

  1. Step 1:

    In original data it seems splines are not joined which is not suitable for sweeping or ribing. So ive start with joining neccesarry splines

  2. Step 2:

    Ive created a circles for profile of frame. Also ive used 20mm radius as example

  3. Step 3:

    As i mentioned in my post i preffer "Sweep" command than "Rib" because of its flexible pharameters. I sweep that circle along our joined guide spline. Also we will redo that step for all guides.

  4. Step 4:

    Iam closing our sweeped surface to make it solid. Its just for example. Also "Thick Surface" command could be used to make them like tubes ( Which is more suitable approach for copper frame )

  5. Step 5:

    I always try to keep symmetrical approach for that kind of works. Because its providing more control over workbench, also less work and more pharametric results. In this step, we are finalizing our frame by mirroring solid body

  6. Step 6:

    And this is our result.

  7. Step 7:

    Also result shot with product tree. As a summary i want to say this is just one option from several appraches to make that chopper frame. CATIA drafters always have much options to achive their goals :) I hope that helps someone. Thanks for your interests.