Hi.
I' looking at your nice model, and I think in 20 cm (you have to re-scale five or six times to that) you haven't many chances to hollow it. And you need to make holes to leave it hollow, that could be ugly in this model when it's printed.
You have to check the minimum thickness for the material you choose, and then watch if there's possible to make it hollow.
There isn't service to make it hollow. I could re-design it and upload it in my shop (now I can't use your file, unless you save it in .ipt), but you have all the work done yet. It don't deserve the effort.
Best you can do is to upload it in some sizes and watch the results. Upload a model is free.
I hope this helps you.
Best regards from the sunny Spain.
Luis