Long drill bits?

We are installing a wood fence and one panel goes between two trees. A post is right beside one of the trees and there is limited room to use a hand drill for the hole for a 4 inch screw. Using a bolt from the side away from the tree won't work because the nut would not allow the fence boards to be attached. On... show more We are installing a wood fence and one panel goes between two trees. A post is right beside one of the trees and there is limited room to use a hand drill for the hole for a 4 inch screw. Using a bolt from the side away from the tree won't work because the nut would not allow the fence boards to be attached. On the boards we can use short screws instead of nails and slowly get those screws in with a ratchet. We have a small battery operated hand drill Black and Decker and that may work on the top cross board and hopefully the middle cross board. I doubt it fits on the bottom cross board. We could use a 2 x 4 x 20 foot board but Lowes and Home Depot no longer carry those. The post are 4 inches so we have two cross boards meeting in the middle of the post on the tree side so one screw for each cross board for a total of 6. Graingers sells some 8 inch bits but they are fairly expensive. over 10 dollars. Ideas?
Update: The tree is unable to be cut down. The post is in a place where we were able to find dirt and not roots. The hole was dirt all the way down to 18 inches. I did not see long bits on Home Depot, I'll check again. Pilot holes before putting up the post was not done. We did not realize at the time the space between... show more The tree is unable to be cut down. The post is in a place where we were able to find dirt and not roots. The hole was dirt all the way down to 18 inches. I did not see long bits on Home Depot, I'll check again. Pilot holes before putting up the post was not done. We did not realize at the time the space between post and tree. The post had to go where there was dirt.
Update 2: We looked on website and in store and never found these: 1/8" bits that are about 8" long
Update 3: The post is already in the ground as of two weeks ago. No pre-drilling. Tree can not be cut it belongs to another owner. Any one else have ideas based on the updates?
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