Minor alteration to STL file

I need a designer who can make minor alterations to an STL file for some components for one of our games. Our designer is on holiday for the next few weeks and we can't edit the STL files so were hoping someone could convert them to be editable in sketch up/CAD, make the alterations and then send us back the amended STL files so we can get the prototypes printed.

The alteration itself involves changing the dimensions of 2 openings on the component and we are testing several designs so we would require 2 STL files. If you see the attached image, we have a series of pen holders (brown objects) with different diameter horizontal openings made for the 2 wands above (White) to plug into . What we need is a further 2 holders created with openings as follows:

6.2mm x 6.2mm
6.1mm x 6.1mm

It is critical that the openings are accurate as we need a very particular amount of play to be built in to the holder.

Hopefully this makes sense but please get in touch if you need any further clarification and If this is something you could do, we'd love to hear from you.

Jamie