I am trying to widen a 15cm wide rear rack to better support a rear bag. The current rack is part of the battery holder and so cannot be simply swapped for a wider rack.
You could try to buy a Monkey Mee, that's a rear carrier add on. U can mount them on most rear racks. They widen the platform to about 30 cm by 25 cm long. And add an upright rack, also about 30 cm high. On the upright part there is a strap with a buckle. This can be easily opened and closed. These are made to support a 20-25 liter backpack or daypack.
As I am interpreting your question, you wish to widen the platform width to fit your rear bag better. The engineer in me would approach it this way:
Build/fabricate a thin platform to place upon the existing platform. This would allow you to customize the width of your platform to conform to your bag perfectly. You could use thin sheet aluminum, plexiglass, wood, or another material. All have advantages and disadvantages. Regardless, round off all sharp edges and corners to prevent injury and wear on your rear bag/attachments. Wood should probably be finished with a sealer/waterproof finish.
Your new platform would need to attach to your existing platform in some way. Options would be:
- Velcro (pretty strong stuff!, plus, because Velcro uses adhesive backing to apply it you can remove it at a later time without leaving any trace). You would likely need to use some preexisting mount points for your rear bag when using Velcro option otherwise your rear bag would be held on to your bike solely by the adhesive used on the Velcro strips (the adhesives are likely the weakest link - the Velcro will stay unless it is defective).
- a few screws (or similar) that affix the new platform to the existing one (this will depend on what material the existing platform is made of, and if this can be done without impacting the battery or other electronics). This option will definitely leave a permanent mark on the existing platform.
- The new platform could be attached to the existing rack using mount points offered on the existing rack (if they exist). For this you would need to add some straps (friction or Velcro) that interface with the existing rack in some manner.
Your rear bag can now attach in one of two ways:
- You can still use attachment points on the existing rack.
- You can create attachment points on your new, custom platform.
Possibly, both preexisting and new attachment points could be utilized. It depends on your specific case.
If you go this (or another) route, add some pictures to your question above showing your solution so others can benefit from any lessons learned and other experiences.