I would resize the bottom corners and not hide the edges:
Code:
local myFrame = CreateFrame("Frame", "myFrame", UIparent, "ButtonFrameTemplate")
myFrame:SetPoint("TOPLEFT")
ButtonFrameTemplate_HidePortrait(myFrame)
myFrame:SetFrameStrata("HIGH")
myFrame:SetWidth(430)
myFrame:SetHeight(220)
local function ResizeCorner(parent, cornerName, newX, newY)
local corner = parent[cornerName]
local oldX, oldY = corner:GetSize()
local L, R, T, B = 0, newX / oldX, 0, newY / oldY
if cornerName:match("Right") then
L, R = 1 - R, 1
end
if cornerName:match("Bottom") then
T, B = 1 - B, 1
end
corner:SetSize(newX, newY)
corner:SetTexCoord(L, R, T, B)
end
ResizeCorner(myFrame.NineSlice, "BottomLeftCorner", 36, 36)
ResizeCorner(myFrame.NineSlice, "BottomRightCorner", 36, 36)