Original Photo:
Masked Photo (in photoshop):
I used a black background as to make it supposedly easier to see the object. But I guess since it's transparent it messes it up.
I was using a photography box for this so the black is near the transparent object.
Kind of, but it won't look right. You can set the black BG as the white point using a levels adjustment layer. This will make the image almost entirely white. Then use the layer blending option and set the layer to only affect darker tones. This will leave you an image that is light and mid tones. Mask out the surrounding BG as required, and put a white layer underneath everything.
The problem is then that your highlights and shadows are primarily inverted from where they should be. And that is because you used a black BG; because it makes it easier to see the translucent material highlights.
The right way to do it would be to take the picture on a white/light BG and use negative lighting (black cards/fabric) for the translucent plastic to reflect. Then, all of the edges/lines that are currently white would instead be black. It's not really important that the BG photograph as pure white, that can be edited... what is more important is the correct tonal relationships in the image as recorded.
Here is a quick edit done as I described (if it is not your image, or you disapprove of me posting it, I will remove it).
I don't see an attempt at a white background either.
Pure white is rather easy to obtain, have a white background (quite far) behind the subject and blow it out with a flash (which is itself behind the subject). The trick part is getting white without getting a bloom on the edges of the subject, so use the histogram to see when you get white and don't go further. Your background will be pure white out of the box.
I agree with the suggestion to use a white backdrop with transparent objects if you need a white background. It will be nearly impossible to apply a realistic whitening through the transparent plastic. Just use any white fabric. You may need to backlight a little or set your lights at a lower angle to shine through the plastic from the side. You'll also need to punch up the contrast a bit.
If you simply want to show transparency, buy a cheap poster frame. Paint the backside of the glass with 6-7 coats of gloss black paint. It makes a high contrast black mirror on the front side. Transparent objects look great on it. Use an antistatic cloth to remove dust. Don't scratch it. Don't use mirror spray because it is silver. You want regular glossy black spray paint.
