You using the LASSO tool or the POLYGONAL LASSO tool?
Right click the LASSO tool. That’ll bring up a small window where you can select POLYGONAL LASSO.
[details=Spoiler]I’m sure there’s an easier way but I can’t recall at the moment because I don’t have the Shop at the moment. If the shape of your coffin is similar to this, then I would draw HALF of it with the POLYGONAL. Like this. Sorry for the crude sketch. MSPaint.
Note: The dashed lines is just there for symmetry.
AFTER you draw the bottom half, copy that layer. On the COPIED LAYER, use CTRL+T, which is the transform tool. When it’s selected [which should happen RIGHT when you press CTRL+T], right click on the selection and a box should show up giving you some transformation options. I think theres one on there that should flip the image so that it mirrors [i.e. look like the dashed lines part].
If not, choose ROTATE 180, then FLIP HORIZONTAL. That should fix it [once again, running off of memory, but I’m sure those options are there].
After you do that, if it’s not aligned right, align it. When you find the right shape that you want, merge it with the other layer [I think CTRL+E?]. If CTRL+E doesn’t work, you can go to LAYERS :r: MERGE DOWN. That should do the trick.[/details]
Hope that helps. Lotta reading, I know, but if you got any more questions or need help, I’ll check back here every now and then.